Manual difficulty has been temporarily restricted to minimum 1024, so it was set to 1024 if you tried to use lower value. I can't remember why - probably there were some issues with bandwidth usage in the past. I will check it and re-enable lower difficulties tomorrow. Vardiff still goes as low as 256 so perhaps it's best to use it for now (it's best to use it anyway unless your miner has some technical issues). It will start with 1024 but should lower the difficulty after few minutes.